This invention relates generally to the deflection of an electron beam in a cathode ray tube (CRT) and is particularly directed to a self-centering deflection yoke assembly for ensuring proper alignment and orientation between the vertical and horizontal deflection coils therein.
The optimal design of a deflection yoke in a CRT requires that the longitudinal centerline of the vertical windings coincide with the longitudinal centerline of the horizontal windings; i.e., the vertical and horizontal coils must be concentrically positioned relative to one another. In addition, the axis of the magnetic field generated by the vertical coil must be perpendicular to the axis of the field generated by the horizontal coil. This ensures that the deflection of the electron beam is along mutually orthogonal axes. A conventional deflection yoke includes two horizontal windings fixedly positioned within a plastic housing called a yoke liner. A vertical coil, which is wound onto two semi-circular halves of a cracked ferrite core, is assembled around the outer periphery of the yoke liner. An undesirable condition known as mismatch occurs when the vertical coil is not concentric with respect to the horizontal coil resulting in the inability of the deflection yoke to provide proper convergence of the electron beam.
When the vertical and horizontal magnetic axes of the deflection yoke are orthogonal, the amount of current induced in the vertical coil due to the magnetic field generated by the horizontal coil, which is referred to as cross talk, is held to a minimum. If cross talk is not minimized, the yoke will generate a distorted raster. Because of the manner in which the horizontal coil is positioned within the yoke liner, its reproducibility is very good. Thus, the orientation of its magnetic axis is quite consistent and predictable, unlike that of the vertical coil. The axis of the vertical coil's magnetic field is determined by the distribution of the wire wound onto the ferrite core. The wire distribution can vary considerably from one coil to another, even when they are wound by the same automatic winding machine. The chief reason for this variation is the relatively wide dimensional tolerance range of the ferrite core. Cores that are dimensionally acceptable but at opposite ends of the tolerance range will exhibit significantly different magnetic axes. The wide range of ferrite core dimensions encountered arises from the manner in which they are manufactured and the ferrite materials from which they are fabricated. Therefore, a means for minimizing cross talk is essential for high quality yoke production.
There are several ways to control both mismatch and cross talk in a deflection yoke. One approach makes use of a notched core which mates with locating ribs molded on the outer surface of the yoke liner. The vertical coil is thus keyed onto the same concentric position for every yoke. However, "locking" the core in a fixed position eliminates the ability to adjust cross talk to a minimum. Therefore, before winding, the ferrite core is precision ground to effectively reduce its wide tolerance range, thereby improving the reproducibility of the vertical coil. This grinding operation is costly and its effectiveness, in general, is marginal. Another approach involving a grinding process is disclosed in U.S. Pat. No. 4,471,261 to Meier et al which contemplates grinding precision grooves in the outer surface of the core, which grooves can then be used as reference surfaces to align the core halves in the winding machine or to position the core halves around the liner. This latter operation requires a special tool which is not an integral part of the yoke liner.
Another approach for controlling mismatch and cross talk involves the use of an interference fit between the vertical coil and the outer contour of the yoke liner. High points, or upraised areas, are provided in the coil by winding several layers of wire in a given area of the ferrite core. The wire distribution must of course be compatible with the desired magnetic output of the coil. As the coil halves are assembled, the high points contact the liner's contour and cause it to deflect slightly. The resilience of the liner is responsible for maintaining the vertical coil's concentricity. Since the angular orientation of the coil is in no way restrained, cross talk can be minimized by slightly rotating the vertical coil with respect to the yoke liner until the vertical and horizontal magnetic fields are orthogonal.
This interference fit approach imposes relatively tight restrictions upon the physical size of the vertical coil. For example, too much wire can make assembly of the core halves impossible, while to little wire can result in mismatch. Achieving the desired magnetic characteristics within these physical limitations can be a tedious task. In addition, another problem associated with the interference fit approach arises from the fact that the wires of the vertical coil are in direct contact with the yoke liner. Rotation of the vertical coil to minimize cross talk can result in a shifting of the wires which changes the deflection yoke's magnetic characteristics.
A variation of the interference fit approach described above utilizes foam-backed tape which is positioned upon and adheres to the outer surface of the yoke liner. Rather than contacting the liner, the vertical coil deforms the foam which acts to center the coil. However, the foam is generally not resilient enough to maintain the concentric alignment of the horizontal and vertical coils. In addition, the foam tends to resist rotation of the vertical coil in attempting to minimize cross talk and, in the process, can cause coil wires to shift or the foam-back tape itself to pull free from the yoke liner.
Yet another method of eliminating mismatch and minimizing cross talk requires a rather sophisticated piece of automatic equipment capable of sensing the horizontal and vertical coil configuration and, with the aid of a computer, automatically positioning the vertical coil in its optimal orientation. Although this approach can be quite effective in optimally positioning the vertical coil, the computer controlled equipment necessary for its implementation is very expensive.
The present invention is intended to overcome the aformentioned limitations of the prior art by providing a self-centering deflection yoke assembly which provides for the concentric alignment of the vertical and horizontal coils over a wide range of dimensional tolerances of the vertical coil magnetic core while permitting rotational displacement between the vertical and horizontal coils to ensure orthogonal alignment of the their respective magnetic fields. The self-centering delfection yoke assembly of the present invention is easily fabricated and assembled, reliably and accurately magnetically aligned by means of a simple manual adjustment without requiring the use of any tools, and is inexpensive.